The invention relates to a method of automatic gain control in a base station, particularly in its receiver, in a cellular radio network.
Automatic gain control (AGC) refers to a feedback control circuit for maintaining amplifier gain, that is, output power level, within a given power range, even though the input power level of the amplifier may vary within a given, even wide, range. If such control does not exist, signal clipping occurs in the amplifier or in a subsequent analog-to-digital converter when it receives too strong a signal, the clipping causing signal distortion. For example in audio equipment, the distortion can be heard as sonic distortion.
A known way to implement AGC in a receiver of a base station is shown in FIG. 3A. A signal received from the radio path by an antenna 112 is applied via a duplex filter 300, which separates reception and transmission directions, to a radio frequency receiver 200. In the radio frequency receiver 200, the radio frequency signal is first applied to a low-noise amplifier 302. The signal is then converted to an intermediate frequency by multiplying 304 it by a signal at a suitable frequency.
A synthesizer 212 arranges the frequencies required by the different units. The clock comprised by the synthesizer 212 can be locally controlled or controlled in a centralized manner from somewhere else, from a base station controller controlling the base station, for example. The synthesizer 212 creates the necessary frequencies by a voltage-controlled oscillator, for example. Then the signal is again amplified in an amplifier 306. The signal is then filtered in a band-pass filter 308 which eliminates frequencies outside the desired frequency band. Next, the signal is amplified or attenuated by a adjustable amplifier 310. Then the signal is converted to the baseband by multiplying 312 it with a signal generated by the local synthesizer 212.
Next, the signal is filtered with a low-pass filter 314 which lets signal frequencies under a given frequency to pass through. The signal is then either amplified or attenuated with a second adjustable amplifier 316. Finally, having been converted to the baseband, filtered and amplified/attenuated, the signal is sampled and quantized in a relatively slow analog-to-digital converter 202, followed by further processing of the signal in a digital form with a digital signal processing processor 232 and its software.
Actual AGC is carried out in block 318, shown in FIG. 3A, where the power level of the analog signal is measured, and said power level is used to decide how the signal will be amplified/attenuated with the adjustable amplifiers 310, 316, for its amplitude to be within the allowed power range of the analog-to-digital converter. The solution works, but is relatively expensive, since it is the gain of an analog signal that is measured and controlled.
Another known way to implement AGC in a receiver of a base station is shown in FIG. 3B. The solution is quite similar to the one shown in FIG. 3A, however, with the following exceptions:
the adjustable amplifier 310 is replaced with a normal amplifier 320,
in lieu of a relatively slow analog-to-digital converter 202, a very fast analog-to-digital converter 322 is used,
in lieu of a separate circuit 318 measuring an analog signal, AGC is implemented by a special application specific integrated circuit (ASIC) 324, which adjusts only one adjustable amplifier 316.
Thus the signal gain measurements required by the control are carried out on a digital signal. The disadvantage of the second solution is that the fast analog-to-digital converter 322 and the special ASIC are expensive. Typically, two samples for each symbol are taken in a slow analog-to-digital converter, while in the fast one, even hundreds of samples are taken for each symbol, i.e. oversampling takes place.
It is an object of the invention to provide a method and an apparatus for implementing the method so as to solve the above problems. This is achieved by the method which is discussed next. A method is provided for automatic gain control in a base station of a cellular radio network, comprising: receiving at least one random access burst transmitted by a subscriber terminal of the cellular radio network on a random access channel; measuring the received power of the random access burst; generating subscriber terminal-specific AGC data on the basis of the received power of at least one random access burst. In the method, subscriber terminal-specific AGC data is stored; a normal burst transmitted by the subscriber terminal on a stand-alone dedicated control channel is received; the gain of the normal burst received is controlled by the subscriber terminal-specific AGC data; an analog-to-digital conversion is performed on the controlled normal burst.
The invention also relates to a network part in a cellular radio network, comprising: a receiver for receiving at least one random access burst transmitted by a subscriber terminal of the cellular radio network on a random access channel; means for measuring the received power of the random access burst; control means for generating subscriber terminal-specific AGC data on the basis of the received power of at least one random access burst.
The base station further comprises: means for storing the subscriber terminal-specific AGC data; a receiver (200) for receiving the normal burst transmitted by the subscriber terminal on a stand-alone dedicated control channel; means for controlling the gain of the received normal burst by the stored subscriber terminal-specific AGC data; an analog-to-digital converter for performing analog-to-digital conversion on the controlled normal burst.
The preferred embodiments of the invention are described in the dependent claims.
The invention is based on utilizing an inexpensive, slow analog-to-digital converter and an intelligent way to implement AGC in place of the expensive solutions described. During the long tail bits of a random access burst, AGC can be performed, and once the AGC data is stored, it can also be used to perform AGC of a normal burst, even on a stand-alone dedicated control channel without the connection being cut. Consequently, the invention implements a kind of fast AGC with slow components.
The method and system of the invention provide several advantages. The structure of the base station receiver becomes simpler, resulting in less costs and an increased reliability of operation.